A variety of services are available today for in-house cleaning of carpets and upholstery. These services utilize equipment for heating cleaning liquid which is conveyed under pressure to and sprayed onto the surface to be cleaned and then vacuum removed from the surface with the soil. This equipment, which often includes an internal combustion engine for driving the cleaning liquid and vacuum pumps, is usually mounted in a panel truck, or van, for ease of transport.
It has been suggested that instead of using a separate heater for heating the cleaning liquid that waste heat from the internal combustion engine be used for that purpose. U.S. Pat. No. 4,109,340 granted Aug. 29, 1978 to L. E. Bates for "TRUCK MOUNTED CARPET CLEANING MACHINE" discloses a system in which the cleaning liquid is passed first through the cylinder block of a liquid cooled, internal combustion engine and then through a heat exchanger which also has engine exhaust gases passing therethrough. U.S. Pat. No. 4,284,127 granted Aug. 18, 1981 to D. S. Collier et al for "CARPET CLEANING SYSTEMS" discloses a similar system which directs the cleaning liquid through a first heat exchanger into which the liquid engine coolant also is directed. The preheated cleaning liquid then passes through a second heat exchanger where it extracts heat from the engine exhaust gases.
Many portable cleaning systems in use today employ air cooled engines for driving the pumps because of the simplicity of that type engine compared to the liquid cooled engine. A disadvantage of air cooled engine systems is that the air used to cool the engine heats up the ambient air. And with the engine and associated apparatus confined within a panel truck, or van, the interior of the truck can become uncomfortably warm when the cleaning system is operated.
Of course, the temperature of the engine cooling air could be advantageously reduced by conveying that air and the cleaning liquid through a heat exchanger to transfer some of the heat in the air to the liquid. With a conventional heat exchanger there must be a significant differential between the temperature of the two fluids passing through the exchanger. Thus, if the cleaning liquid is to be heated to, say, 100.degree. F., the temperature of the cooling air exiting the heat exchanger likely would be around 120.degree. F. This air, then, continues to waste some heat and continues to produce uncomfortable conditions inside the truck.